sftp: fix range downloads in both SSH backends

When asking for the last N bytes of a file, and that size was larger
than the file size, it would miss the first byte due to a logic error.

The fixed range parser is now made a common function in the file now
renamed to vssh.c (from curl_path.c) - used by both backends.

Unit test 2605 verifies the parser.

Reported-by: Stanislav Fort (Aisle Research)
Closes #19460

tests/data/Makefile.am

@@ -265,7 +265,7 @@ test2400 test2401 test2402 test2403 test2404 test2405 test2406 \
 \
 test2500 test2501 test2502 test2503 \
 \
-test2600 test2601 test2602 test2603 test2604 \
+test2600 test2601 test2602 test2603 test2604 test2605 \
 \
 test2700 test2701 test2702 test2703 test2704 test2705 test2706 test2707 \
 test2708 test2709 test2710 test2711 test2712 test2713 test2714 test2715 \

tests/data/test2605

@@ -0,0 +1,20 @@
+<testcase>
+<info>
+<keywords>
+unittest
+sftp
+</keywords>
+</info>
+
+#
+# Client-side
+<client>
+<features>
+unittest
+sftp
+</features>
+<name>
+Curl_ssh_range unit test
+</name>
+</client>
+</testcase>

tests/data/test637

@@ -35,7 +35,7 @@ for ssh test
 # Verify data after the test has been "shot"
 <verify>
 <errorcode>
-36
+33
 </errorcode>
 </verify>
 </testcase>